Electrochemical Passivation and Modification of c-Si surfaces

Abstract

This chapter addresses the electrochemical passivation of Si surfaces by hydrogen, small organic molecules and ultra-thin polymeric layers which is not yet a standard technique in Si solar cell preparation. The electrochemical surface conditioning leads to different surface structures compared to the wet-chemical techniques (Chapter 3). The electronic properties of the interface depend strongly on these surface morphologies and consequently it is important to measure and control their changes during the electrochemical processing. Therefore, pulsed photoluminescence (PL) spectroscopy is applied as fast and non-destructive method to monitor in-situ and ex-situ the electronic surface properties during electrochemical oxidation, hydrogenation, and grafting of organic molecules and ultra-thin polymeric layers. The additionally used in-situ surface photovoltage (SPV) provides information on the surface charge during the wet-chemical and electrochemical processing. Unusual low concentration of recombination active defects at Si:H surfaces, Si(111) and Si(100), can be obtained after electropolishing in the current oscillating regime in diluted HF solutions. The passivation by hydrogen is influenced by the applied potential, the current flow, the temperature and the solution composition, where nitrogen bubbling of the solution is an important step to enhance the surface passivation. PL investigations of the organically modified surfaces show that a slightly higher defect concentration at the interface (typical by a factor of 2) is usually observed. However, organically modified Si surfaces have extremely long time stability versus oxidation in ambient air, especially after grafting of 10-carboxydecyl groups via hydrosilylation which shows an ideal Si surface passivation with respect to interface recombination losses.